You can find good explanations of exactly what GLAST is in most of the above references.

What I'll summarize here are just some of the main objects and phenomena that GLAST is expected to help observe and study.

Gamma-ray bursts

Gamma-ray bursts (GRBs) have been discussed here several times, such as here and here. It is generally agreed that there are several different events that can cause a GRB, and a fair amount is known about the phenomenon already. For instance, the subtype known as a "long" GRB is thought to result from supernova explosions in which a high-energy jet of particles and radiation is emitted in a narrow beam that happens to point in our direction. But as yet we haven't measured the complete spectrum of energy from a GRB of any type. This spectrum can range from a few KeV to hundreds of GeV, and knowing it in detail would help determine the nature of the associated event much more accurately.

Dark matter

The visible universe that consists of luminous objects like stars and galaxies is composed of baryonic matter (mainly protons and helium nuclei). There may be at least as much baryonic matter in the form of diffuse gas that we cannot see. (Recent observations here and here.)

Yet it is essentially certain that the universe actually contains about four times as much matter that we can't detect at all (except by its gravitational effects) as all that baryonic matter put together. This is the dark matter. There are many theories about what this dark matter consists of, but in one of the main classes of theories, the matter consists of "weakly-interacting massive particles" (WIMPs). In most such theories, WIMPs can annihilate each other in pairs, giving off copious quantities of gamma-rays (among other things).

If some such theory accounts for a portion of the dark matter, GLAST will make it possible to estimate properties of WIMPs (e. g. their mass) by observing gamma-rays from locations where dark matter is expected to be concentrated, such as in the center of the Milky Way. This kind of information will complement and help corroborate observations made at the Large Hadron Collider, in which some kinds of WIMPs (if they exist at all) are expected to be created.

Solar gamma-rays

Although our Sun is a relatively weak source of gamma-rays compared to almost everything else mentioned here (even weaker than the Moon, where gamma-rays are produced when cosmic rays strike the surface), several solar events, such as flares and coronal mass ejections do produce gamma-rays. So GLAST will help us better understand solar events of this kind.

Supernova remnants

A large class of gamma-ray bursts are associated with the initial blast of a supernova event, and the gamma-rays from such bursts subside in a matter of minutes. But other gamma-rays may originate by other mechanisms from the supernova remnant long after the original event. Gamma-rays are thought to be produced in such remnants due to particles being accelerated to high energies in the blast and subsequently generating shock waves in the interstellar medium. The shock waves themselves are reasonably well understood, but how the particles are actually accelerated by the supernova blast needs much more elucidation, which GLAST can provide.

Pulsars

Another part of the remnant left over after a supernova is either a stellar-mass black hole, or else a rapidly spinning neutron star. Such a neutron star will produce jets of electromagnetic energy, usually at radio frequencies, and when the Earth is lined up with the jet the object is called a pulsar. These also emit gamma-rays. Since neutron stars are extremely small and dense, they have intense magnetic fields near their surface, and the fields reveal a lot about the nature of matter in the neutron star. The strong fields also convert gamma-rays into electron-positron pairs, so the overall gamma-ray spectrum can give us information about the object's magnetic fields, and about surface features that cause gamma-ray emission.

Supermassive black holes, active galactic nuclei, quasars, blazars

Supermassive black holes are thought to exist at the centers of most or all galaxies. We can estimate that they have masses ranging from 105 to 1010 solar masses, yet there is a great deal more we would like to know about them, such as the process by which they form. (See here.) Most supermassive black holes are thought to be circled by an accretion disk of matter which has been attracted by the object's extreme gravity.

Depending on the amount of matter in the disk, large amounts of energy may be released as the matter falls into the black hole. Our own Milky Way has a smallish object of this sort, with a correspondingly small accretion disk. But if there is much more mass in the disk, one has a bright object called an active galactic nucleus (AGN). AGNs were more plentiful in the early days of the universe, before most of the available nearby matter had been consumed by the black hole, and especially active objects of this kind, usually at great distances, are called quasars.

Like supernovae (from which stellar-mass black holes or neutron stars are formed), supermassive black holes may emit powerful relativistic jets of particles and energy. If such a jet is pointed in our direction, we see an especially bright source, called a blazar. Most of the emitted electromagnetic energy from all these objects is in the gamma-ray part of the spectrum.

So one of the main objectives of GLAST is to measure how this spectrum varies over time, in order to get a better understanding of what is actually going on. For instance, there could be additional confirmation of a model of the relativistic jets as described in this recent research, and perhaps evidence as to whether the particles in the jets are protons or electrons.

Cosmic ray origins

As discussed in detail here, we are finally beginning to clear away some of the mystery surrounding the most energetic ultra-high-energy cosmic rays (UHECRs). But there's a lot more we'd like to know, such as whether these rays are mostly made up of relativistic protons, and what sort of process creates them in the first place. Gamma-rays are produced when UHECRs interact with interstellar gas and dust, so GLAST may be able to give us more information about UHECRs.

Primordial black holes

It is generally suspected that many small black holes (with masses covering a wide range, but much less than the mass of a star) could have been produced in the big bang. These are called primordial black holes. Their existence hasn't yet been confirmed. But Stephen Hawking made a strong case that any black hole will slowly emit weak electromagnetic radiation, due to quantum effects and called Hawking radiation. This radiation should be too weak to be directly observable. However, small primordial black holes should eventually evaporate completely by this process, and at the end disintegrate in a burst of gamma-rays. This is all rather conjectural, but if it happens, it may contribute to a continuous gamma-ray background that can be detected.

Cosmic gamma-ray background

In addition to discrete gamma-ray sources such as GRBs and supernova remnants, there is a diffuse background of gamma-ray photons, much like the cosmic microwave background (CMB), only vastly more energetic. Some of this background may be due to UHECRs, very distant and very powerful (TeV range) gamma-ray sources, or primordial black holes. But who knows what other kinds of sources might be out there? There will probably be some surprises, as well as a lot of useful information to be deduced, just as happened with the CMB.

Possible breakdowns of special relativity

Heading even further into speculative territory, various theorists of quantum gravityspecial relativity (as well as general relativity too) may break down under various conditions. For example, the speed of light might not be an exact constant, but might instead vary by a slight amount according to the energy carried by individual photons.

Thus not all gamma-ray photons from a GRB would need arrive at precisely the same time, and so any pattern in this radiation would be shifted very slightly depending on what part of the gamma-ray spectrum is observed. Even if the shift is as little as 1/1000 of a second (for photons that may have been travelling for billions of years), GLAST should be sensitive enough to detect the shift. That would certainly be quite a surprise if found.

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